The present disclosure relates to a method and an apparatus for detecting foam on a liquid surface in a vessel.
Such systems for detecting foam on a liquid surface in a vessel are particularly useful in the field of automatic analysers which comprise pipetting systems for the exchange of predetermined amounts of liquid between vessels, or containers, which are automatically handled in such automatic analyser systems. The pipetting of a liquid sample, or an analyte, from a respective vessel is carried out by a pipette which is slightly immersed with its pipette tip into the liquid, and a predefined volume of the liquid is sucked into the pipette. If there is a foam layer on the liquid, it might happen that the end position of the tip of the pipette is inside the foam layer, and thus a considerable amount of foam will be sucked into the pipette during the pipetting period. This can lead to a wrong analytical result of the automatic analyser.
Since the filling level of the liquid generally changes from vessel to vessel, technical solutions have been already developed to detect the liquid surface level. A collimated light is directed to the surface of the liquid. The preferred incident angle is between 10° and 20°. A semiconductor photo sensor is used as a CCD-photo detector which is directed to the surface of the liquid. Scanning of the incident angle or movement of the complete detection system can give more information about the liquid surface. Air bubbles can be detected by the disturbance of reflectance. The optical detection system can be combined with a capacitance measurement for liquid level detection. The systems of the prior art do not involve image acquisition and image processing for determining the extension of the foam.
A dynamic fluid level and bubble inspection for quality and process control relates to fast image processing for the detection of bubbles in a liquid and gives information on algorithms to characterize the bubbles. However, foam on the surface is not considered and images are taken in transmission and not from the top of the vessel. In a similar, images of foam are taken through the vascular wall of the vessel. There is no description for taking images from the top of the vessel.
Therefore, there is a need to provide an apparatus and method for detecting foam on a liquid surface in a vessel, which can be implemented in an automated liquid pipetting system and which can be executed to automatically distinguish between foam areas and non-foam areas on the liquid surface in a sensitive manner in order to provide information as a basis for the decision whether the vessel is prepared for pipetting liquid therefrom.